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The formation of the island of Surtsey over 3.5 years, remains one of the best-documented volcanic, island-forming eruptions to date. The basaltic submarine volcanic activity was detected on November 14, 1963, where ocean depth was 130 m prior to the eruption at the southern end of the Vestmannaeyjar archipelago. The eruptions occurred in several phases involving explosive and effusive activity, including the initial submarine phase on November 12–13, 1963. Separate phases of subaerial volcanic activity occurred during November 14, 1963–January 1964, January–April 1964, April 1964–May 1965, May–October 1965, December 1965–August 1966, and August 1966–June 1967. Seismic data quality from this period is inferior compared to that of modern monitoring systems. Four permanent seismic stations were operated in Iceland at the time, whereof only two, located at 115 and 140 km distance, had the sensitivity to record tremor from Surtsey. Nevertheless, the scanned analog seismograms (http://seismis.hi.is/) show that the eruptive activity was accompanied by considerable seismic activity, both earthquakes, and volcanic tremor. Earthquakes were primarily associated with changes in vent location. Both spasmodic and harmonic tremor was identified, both with low (\>3 Hz) and higher (3–5 Hz) characteristic frequencies. The results indicate a complicated relationship between tremor and magma flow rate or style of activity. During the explosive eruption, the highest magma flow rates occurred in the first 10–20 days, a period with little observed tremor. The highest tremor is observed in December 1963–March 1964, after the discharge rates had dropped substantially, and on a timescale of hours-to-days, no clear relationship between tremor and eruption style is observed. The same applies to the effusive activity, where no seismic tremor was observed during most of the effusive eruption of Surtungur, despite the fact that magma flow rates were ∼3 times higher than during later phases where some tremor was observed.

The basaltic effusive eruption at Mt. Fagradalsfjall began on March 19, 2021, ending a 781-year hiatus on Reykjanes Peninsula, Iceland. At the time of writing (January 7, 2022), no eruptive activity has been observed since September 18, 2021. To monitor key eruption parameters (i.e., effusion rate and volume), near-real time photogrammetric monitoring was performed using a combination of satellite and airborne stereo images.By late September 2021, 32 near real-time photogrammetric surveys were completed, usually processed within 3–6 hours. The results are a significant achievement in full-scale monitoring of a lava flow-field providing temporal data sets of lava volume, thickness, and effusion rate. This enabled rapid assessment of eruption evolution and hazards to populated areas, important infrastructure, and tourist centers.The lava pathways and lava advancement were very complex and changeable as the lava filled and spilled from one valley into another and short-term prediction of the timing of overflow from one valley to another proved challenging. Analysis of thickness maps and thickness change maps show that the lava transport into different valleys varied up to 10 m3/s between surveys as lava transport rapidly switched between one valley to another.By late September 2021, the mean lava thickness exceeded 30 m, covered 4.8 km2 and has a bulk volume of 150 ± 3 × 106 m3. Around the vent the thickness is up to 122 m. The March–September mean effusion rate is 9.5 ± 0.2 m3/s, ranging between 1–8 m3/s in March–April and increasing to 9–13 m3/s in May–September. This is uncommon for recent Icelandic eruptions, where the highest discharge usually occurs in the opening phase. This behavior may have been due to widening of the conduit by thermo-mechanical erosion with time, and not controlled by magma chamber pressure as is most common in the volcanic zones of Iceland.

Iceland has the largest variety of subglacially formed volcanic edifices worldwide, given the extensive glacial cover during the Pleistocene and its frequent volcanic activity. As substantial parts of the volcanic zones are presently ice-covered, eruptions beneath glaciers are common. Phreatomagmatic activity and flood deposits have been hypothesised for shallow subglacial fissure eruptions, at or within a glacial margin. However, to date, no historical examples that did not immediately break through the ice, resulting in dry magmatic activity, have been directly observed. Also, at dynamic ice-margin settings, no extensive resultant formations from shallow subglacial fissure eruptions formed in older historic eruptions have been studied until now. The final fissure from the 1783–84 CE Laki basaltic flood lava event in the Síða highlands of Iceland, fissure 10, provides a perfect natural laboratory to understand the eruptive dynamics of a shallow subglacial or intraglacial fissure eruption. Fissure 10 is a 2.5 km long formation, which constitutes the final phase of activity on the 29 km long Laki crater row, formed as eruptive activity from the Laki eruption propagated under Síðujökull, an outlet glacier from the Vatnajökull ice-cap. The resultant eruptive sequences display evidence of the increasing influence of ice when traced along strike from SW to NE, with the eruption transitioning to a predominantly phreatomagmatic phase with increasing degrees of lateral confinement. The sequence is dominated by volcanoclastic units, formed by multiple phreatomagmatic and magmatic phases suggestive of fluctuating water levels, intercalated with hackly jointed intrusions, hackly jointed lobate lava flows and debris flows. Repeating units of agglutinated spatter and spatter-fed lava flows cap the sequence, suggesting decreasing influence of external water with stratigraphic height and towards the end of the fissure’s eruptive activity. A thin layer of glacial till coats the top of the fissure 10 sequences. The margin of Síðujökull has since fully receded from the formation. Our model for the eruptive dynamics of the northern Laki fissure 10 formation is based on field mapping, a drone photogrammetry survey, petrological observations and EMP analysis of glassy tephra and lava selvages to gain a full understanding of the activity and how eruptive activity progressed. The Laki eruption benefits from a wealth of previous studies on the magmatic phases from the other 9 subaerially eruptive fissures, to the SW of fissure 10, allowing for the effects of the glacier on this fissure’s activity to be isolated and defined. Fissure 10 allows for an approximate reconstruction of the ice margin and glacier slope at the time of eruption, adding valuable information on the extent of the glaciers in SW-Vatnajökull in the late 18th century, and during the Little Ice Age. These shallow subglacially erupted deposits are the only fully accessible intraglacial eruptive vents, from a known historical eruption, on Earth. Detailed mapping and petrological analysis of deposits like these is important for interpreting landforms in paleo-ice margins, where transitional activity occurs.

Gjálp is a hyaloclastite ridge situated beneath the western part of the ~8000 km2 Vatnajökull ice cap, located midway between the subglacial calderas of Grímsvötn and Bárdabunga volcanoes. The tephra erupted at Gjálp has affinities fitting with the Grímsvötn volcanic system while the associated seismicity and unrest preceding the eruption suggest that the eruption was caused by lateral magma flow from Bárdarbunga. Eruptions occurred at Gjálp in 1938 and 1996 but only the 1996 eruption is thought to have broken through the ice. The 1996 eruption was first detected on the 30th of September at about 22:00 GMT by the onset of seismic tremor; the following day heavily crevassed ice cauldrons were noticed. Around 30 hours after detection of the tremor the eruption broke through the ice sheet. The eruption lasted for 13 days, during which a 6-7 km long subglacial, hyaloclastite ridge was formed. The subglacial eruption melted large volumes of ice that accumulated within the Grímsvötn caldera until early November, when it was released in a major jökulhlaup, destroying bridges and damaging roads. In comparison with the subglacial eruption the subaerial part was relatively modest. The style of activity was mostly Surtseyan and the tephra erupted is mildly intermediate in composition.The tephra fall began on October 2 and continued intermittently until October 13. The first tephra was seen at 05:18 on October 2. By 08:50 the largest explosions threw tephra about 1 km above the ice surface and the plume rose to 4-4.5 km above sea level. This tephra was carried north and north-northeast across North and Central Iceland and was detected as far as 250 km from source. On October 3 the plume was reported to have reached 8-9 km a.s.l. Tephra was also dispersed to the east and south and most of the tephra accumulated on the Vatnajökull glacier. During the eruption, repeated snow fall caused layering within the tephra deposit. In the following year samples were collected from the tephra fall area on the glacier. These consist mostly of snow cores with tephra thickness ranging from dm to mm. The samples were processed to estimate the tephra volume and to create a dispersal and isopach map. The tephra layer deposited on the glacier is volumetrically only a few percent of the bulk volume (~0.7 km3) of the subglacial ridge formed in the 1996 eruption.

Tuyas are basaltic to intermediate glaciovolcanic edifices, formed in a body of meltwater within an ice sheet, in an ocean or a lake. The most common tuya stratigraphy consist of a lowermost layer or a mound of pillow lava, overlain by hyaloclastite tuffs and capped by a layer of subaerially-formed, horizontally bedded, lava flows. The parts of the lava flows more distant from the vent are built on flow-foot breccias, with the transition from subaerially-formed lava flows and breccias being a distinct stratigraphic boundary: the passage zone. The elevation of the passage zone marks the water level in the englacial lake into which the evolving tuya was built. At many locations the elevation of the passage zone appears to vary considerably from one location on a tuya to another. Some tuyas are elongated. One idea is that the elongation is predominantly in the direction of ice flow at the time of eruption.By studying tuyas through aerial photography, satellite imagery and ground observations, the edifices variations in the elevation of the passage zone can be studied. This provides information on the eruption processes and environmental conditions at the time of formation. We have analyzed the variation of passage zone elevation with distance along strike of a selected set of tuyas in Iceland. These include Bláfjall, located in Northern Iceland. It was formed within a Pleistocene ice sheet a continuous, prolonged eruption, or in a series of eruptions, closely spaced in time. The lava cap reaches a maximum thickness of approximately 100 m but is only a few meters to a few tens of meters thick on average, showing clear signs of influence from the ice sheet. Apparently, both the thickness of the ice sheet and the direction of ice flow direction exerted major control on the height and elongation of the Bláfjall tuya. The eruption took place well to the north of the ice divide at the time, and the flow of ice was predominantly from south to north, with the elongated structure of the tuya oriented parallel to the flow of the ancient glacier. The thickness of the lava cap is greatest in the north part and generally decreases towards south. This is despite the fact that the elevation of the mountain increases southwards. This indicates that the northern part is mostly formed by an advancing lava delta, propagating in the direction of ice flow and that the level of the water body present at the end of the advancing lava delta become progressively lower towards north. This suggests a sloping ice sheet at the time of formation, or possibly a receding ice sheet, leading to gradual thinning with time as the eruption progressed.